Scala multi-root tree or forest

forest.scala

package com.signalpath.model

import scala.collection.mutable

class Forest[A]()(ordering: Ordering[A]) {
  val tree = new mutable.LinkedHashMap[Option[A], mutable.ListBuffer[A]]()

  def addNode(parent: Option[A], node: A): Unit = {
    val children = tree.get(parent).map(_ += node).getOrElse(mutable.ListBuffer[A](node))
    tree.put(parent,children.sorted(ordering))
  }

  def getChildren(parent: Option[A]): List[A] = {
    tree.getOrElse(parent, List.empty[A]).toList
  }

  def getRootNodes(): Set[Option[A]] = {
    tree.keySet.toSet
  }
}